Porcelain coated metal boards are recognized as having many advantages in comparison to organic plastic boards for use in electronic devices. The porcelain coated metal boards, hereafter referred to as porcelain boards, are inherently more resistant to physical damage and chemical attack than organic plastic boards. The porcelain boards can be employed over a wider range of temperatures than organic plastic boards and are particularly useful at elevated temperatures.
Certain problems have, however, been encountered with procelain boards which have heretofore limited their full potential utilization. The design, rules, and practices of the electronic industry require that circuit assembly be as compact as possible. This requires that the electrical circuitry and the components which comprise the electronic devices be packed as closely as possible on the circuit boards. To reduce the physical size and to facilitate the assembly to the devices it has become standard practice to place electronic components on both the face and reverse surfaces of circuit boards. This technique requires a large number of apertures through the boards for making electrical connections between the face and reverse surfaces of the circuit boards.
Many of the electronic components comprising the circuitry are now formed by using thick film techniques in which the desired components or circuits are printed directly on one or both surfaces of the circuit boards. In order to obtain maximum packing and accurate reproduction of electronic elements by thick film techniques, it is necessary that the areas of the circuit boards which are printed be as flat as possible. The flatness requirement has not proven to be a problem with the organic plastic boards as they can readily be manufactured in a flat configuration and apertures can be drilled or punched through the board without distorting the overall flatness. The porcelain boards heretofore available, especially those having apertures for making electrical connections between the face and the reverse surfaces thereof have, however, not been sufficiently flat, especially around the apertures, for use in thick film processes. This inherent lack of flatness of the porcelain boards heretofore available restricts the relative amount of surface area which can be utilized for printed circuits. This causes significant problems in reducing the physical size of the circuits formed by these porcelain boards in comparison to similar circuits formed on organic plastic boards.
Certain unique problems have also been encountered in making electrical connections between the face and the reverse surfaces of porcelain boards as compared to the organic plastic boards. When an organic plastic board is employed, the organic plastic being comprised of a dielectric material provides the required insulation for the electrical leads which passes through the board. When it is desired to make a direct connection between circuitry on opposite sides of an organic plastic board, plated through holes or the like can be provided by well-known techniques.
The provision of connections through porcelain boards, however, presents considerably more problems. Porcelain boards have conductive metal cores. If the metal core is used as a common ground plane, it is essential that the leads from the electronic components do not inadvertantly contact the metal core and short to ground. Even if the metal core is not used as a common ground plane, contact by the exposed leads of electronic components with the conductive metal core can cause shorts between components mounted on the board through the metal core of the board. It is therefore recognized that the walls of the apertures through the porcelain boards must be well insulated with a dielectric material.
The prior art techniques of providing connections between the surfaces of porcelain boards results in a considerable waste of valuable surface area on the face and referse sides of the board. Typically the amount of area required for making a connection through porcelain boards is about 10 times that required for an organic plastic board. In addition the number of separate steps required to provide the connections as well as certain problems which are encountered when insulating the metal core as noted above significantly add to the manufacturing cost of porcelain boards in comparison to organic plastic boards.
It would be a substantial advantage if a method of manufacture could be provided for porcelain boards in which connections between the face and the reverse surface thereof could be made with fewer, less costly process steps and which would provide a finished porcelain board having more usable surface area.